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From Erick Erickson <>
Subject Re: In memory index (current status in Lucene)
Date Mon, 01 Jul 2013 10:30:22 GMT
Hey Emma! It's been a while....

Building on what Steven said, here's Uwe's blog on
MMapDirectory and Lucene:

I've always considered RAMDirectory for rather restricted
use-cases. I.e. if I know without doubt that the index
is both relatively static and bounded. The other use I've
seen is to use it to index single documents on-the-fly for
some reason (say complex processing of a single result)
then throw it out afterwards.

How are things going?


On Fri, Jun 28, 2013 at 5:36 PM, Steven Schlansker <>wrote:

> On Jun 28, 2013, at 2:29 PM, Emmanuel Espina <>
> wrote:
> > I'm building a distributed index (mostly as a reasearch project for
> > school) and I'm evaluating indexing the entire collection in memory
> > (like google, facebook and others have done years ago). The obvious
> > reason for this is performance considering that the replication will
> > give me a reasonably good durability of the data (despite being in
> > volatile memory).
> >
> > What is the current status of Lucene for this kind of indexes?
> > RAMDirectory in it's documentation has a scary warning that says that
> > "is not intended to work with huge indexes", and that sounds more like
> > it is an implementation for testing rather than something for
> > production.
> >
> > Of course there is no real context for this question, because it is a
> > reasearch topic. Testing it's limits would be the closest to a context
> > I have :p
> You could consider MMapDirectory, which will end up putting the active
> portions
> of the index in memory (via the filesystem buffer cache).
> The benefit is that you don't completely destroy the Java heap
> (RAMDirectory causes immense
> GC pressure if you are not careful) and you don't have to commit all of
> your ram to index usage all the time.
> The downside is that if your working set exceeds the amount of RAM
> available for buffer cache, you will get silent performance degradation as
> you fall back to disk reads for the missing blocks.
> Maybe this is OK for your use case, maybe not.
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